In the past, tubulars, such as pipe joints for use in oil fields, were constructed from metallic materials. More recently, tubulars also have been formed of composite materials such as fiberglass reinforced plastic to take advantage of the special characteristics of such composite materials. For example, composite materials are more inert than metallic materials to corrosive attack.
Pipe joints used in and around oil wells should be sufficiently durable to withstand rough handling and abuse. Whether the pipe joints are suspended in the well or used above ground, the pipe joints and connectors of the pipe joints must be strong and rugged. The connectors of the pipe joints should be able to provide a reliable seal under a variety of harsh environments such as exposure to high temperatures, high pressures, stresses, and corrosive agents.
The typical threaded composite pipe joint is designed so that when the joints are made-up (screwed together), the joints are rotated to a certain torque to join the tubulars together so that the tapered threads form a pressure tight seal. But during make-up the pipe joint is often subjected to excessive torque, which can cause damage to the threads of the connectors of the pipe joints. This damage is referred to as galling of the threads.
Galling of the threads compromises the seal and weakens the strength of the connection between the tubulars. For example, a typical composite pipe connector for a three-inch pipe may be threaded together with torque in the range of one-hundred-and-seventy-five (175) foot-pounds to two-hundred-and-seventy-five (275) foot-pounds. Lower torque does not provide a pressure tight seal; higher torque results in galling of the threads, which causes a loss of seal efficiency.
Galling also causes problems during break-out (unscrewing) of the pipe joint. Galling may make break-out difficult or impossible and often prevents the connectors of the pipe joint from being made-up again, which renders the affected pipe joints unusable. Galling is particularly detrimental to down-hole tubing, which often undergoes multiple make-ups and break-outs during its lifetime. These multiple make-ups and break-outs can also result in thread wear that is detrimental to the useful life of down-hole tubing.